This application is for the renewal of CA098286, a multicentered, double-blind, randomized, placebo controlled trial of supplementation with vitamin D and/or calcium for the prevention of colorectal adenomas. The study builds on extensive epidemiological and experimental data indicating that both vitamin D and calcium exert anti-neoplastic effects in the large bowel. The study involves 11 clinical centers in the US, a pathology center and a coordinating center. Each subject has at least one large bowel adenoma removed in the 4 months prior to study entry, with complete colonoscopic visualization of the colorectal mucosa and no known polyps remaining in the bowel. Subjects at risk for toxicity from study agents (e.g. with a recent history of kidney stones) are excluded, as are those with a likely need for the study treatments (e.g. with osteoporosis). Participants agree to avoid taking study agents outside the trial. Suitable subjects are randomized in a 2 x 2 factorial manner to calcium carbonate (1200 mg calcium/day), vitamin D (1000 IU/day), both agents, or placebo. We offer women the choice of being randomized to calcium alone or calcium plus vitamin D. Colonoscopic follow-up occurs at either 3 years or 5 years after the qualifying exam, as planned by each subject's gastroenterologist. As safety measures, serum calcium, creatinine and 25-(OH) vitamin D levels will be obtained at baseline, and during the treatment period. The main endpoint of the study is one or more neoplastic polyps of the bowel on followup;assessment of effects for advanced lesions will be included in secondary analyses. Under conservative assumptions regarding event rates and study conduct, we will have greater than 89% power to detect a 20% reduction in adenoma recurrence with vitamin D versus placebo, and 85% power to detect a 25% reduction in recurrence with calcium plus vitamin D versus calcium alone. Other analyses will also have good statistical power. During the first 5 years of funding, enrollment has proceeded almost exactly according to schedule;we will meet or exceed our target enrollment on November 30, 2007. To date, subjects have shown excellent compliance with study procedures and agents, and few subjects have had to stop randomized treatment because of adverse events (e.g. kidney stones) or a need for the study agents (e.g. osteoporosis). We now request funds to complete the planned treatment and follow-up for all 2,200 randomized subjects, prepare for the first efficacy analyses, and implement a post-treatment follow-up.